Effects of Microstructure on the Mechanical Properties and Stress Corrosion Cracking of an Al-Zn-Mg-Sc-Zr Alloy by Various Temper Treatments

Abstract

The high strength Al-Zn-Mg alloy used in the aerospace industry is strengthened by coherent G.P. zones and semicoherent 0 phase. However, this series of aluminum alloys are susceptible to stress corrosion cracking (SCC), particularly when aged to the peak-aged state of T6 temper. In this study, the effect of microstructure on mechanical properties and stress corrosion cracking of the alloy was investigated for alloys tempered to T6, RRA, Two-Step and T7 conditions by tensile test in air, SCC test and polarization test in a 3.5%NaCl solution. It is shown that the improvement in SCC resistance correlates very well with the size of matrix precipitates and grain boundary precipitates. T7 temper can produce larger sizes of both the matrix precipitates and grain boundary precipitates than that of T6, RRA and Two-step tempers, causing a decrease in the length and density of dislocation lines, which results in the decrease of stress concentration at grain boundary and greatly improving the SCC resistance. The addition of Sc and Zr to high strength Al-Zn-Mg alloys has been found to simultaneously improve the tensile strength and SCC resistance.

Quantitative study of precipitates in an Al–Zn–Mg–Cu alloy aged with various typical tempers

“…Also, the sample with a larger precipitate volume fraction tends to have a relatively higher electrical conductivity. This is consistent with that reported in references [20][21][22][23][24][25][26][27]. The evolution for tensile strength of the alloy with the precipitate size suggests a predominant orowan bypassing mechanism for the over-aging states.…”

Microstructural, mechanical and electrochemical behaviour of a 7017 Al–Zn–Mg alloy of different tempers

“…It is reported in several literatures [33][34][35][36] that stress corrosion cracks invariably initiates from the base of the pits and or from the intergranularly corroded/denuded regions. A few literature [37][38][39] has also dealt with investigation of electrochemical behavior and its effect on SCC behavior has been studied many researchers. However, it is very much essential to study the electrochemical behavior of the 7xxx alloys in order to assess its corrosion resistance and also to acquaint with the electrochemical data e.g.…”

Environmental Effects on the Incubation Time Characteristics in Stress-Corrosion Cracking

“…It was found that an increase in the chloride ion content of environment up to 0.6M enhanced the SCC growth rate, and the cracking was slower at higher concentrations. Great efforts have been made to increase the stress corrosion resistance (SCR) of high strength aluminum alloys by optimization of microstructure via heat treatment [93][94][95][96][97][98][99][100][101][102][103][104][105][106][107][108][109].…”